MOISTURE EXCLUDING EFFECTIVENESS AND 

WEIGHT OF AIRCRAFT FINISHES ON 

PAPREG AND ON PLYWOOD 

May 1944 

This Report is One of a Series 

Issued In Cooperation with the 

ARMY-NAVY-CIVIL COMMITTEE 

on 

AIRCRAFT DESIGN CRlTERlA 

Under the Supervision of the 

AERONAUTICAL BOARD 

No. 1398 

UNITED STATES DEPARTMENT OF AGRICULTURE 

F O R E S T S E R V I C E 

F O R E S T P R O D U C T S L A B O R A T O R Y 

Madison, Wisconsin 

In Cooperation with the University of Wisconsin 
- - - - - 

MOISTURE-EXCLUDING EFFECTIVENESS AND WEIGHT OF 

AIRCRAFT FINISHES ON PAPREG AND ON PLYWOOD1 

By F. L. BROWNE, Principal Chemist, 

and A. C. SCHWEBS, Associate Chemist 

Introduction 

This study was part of a broader investigation of the finishing characteristics of papreg, the laminated paper plastic developed at the Forest 

Products Laboratory, and of aircraft finishes for plywood. The work was started 

under authorization of the Army-Navy Aeronautical Directional Committee, but the authorization was withdrawn before all the test panels originally programmed 

Could be prepared. After the curtailment, those tests that wore already under 

way were continued, but no further test panels were prepared. The results 

presented in this report, therefore, represent a preliminary survey rather than 

an adequate study of the subject. 

Papreg presents a glazed, nonporous surface in contrast to the vascular 

surface of wood. Finishing materials do not penetrate into papreg, which 

therefore has no need of the sealers or primers required to hold out coatings 

on wood. Likewise papreg, being immune to the corrosion to which metals are 

subject, needs no inhibitive primers. The presumption is that finishing systems 

for papreg may be simpler and lighter in weight than those for wood or metal. 

Finishes must cling to papreg entirely by specific adhesion without the 

aid of the mechanical adhesion available on wood. Under such circumstances, 

the fact that the surface of papreg consists largely of hardened phenolic resin 

makes it necessary to consider carefully whether requirements of compatibility 

with the resin may not restrict the nature of the vehicle in finishes suitable 

for papreg. 

Papreg resembles wood more closely than it does metal in that papreg 

absorbs moisture and swells, though the amount of absorption under comparable 

conditions is less than that of wood. At edges of sheets where papreg has 

been cut, moisture is absorbed more rapidly than it is through surfaces that 

were in contact with the platens of the press in which the papreg vas formed. 

It is assumed for the present that finishes, suitable for papreg to be exposed 

to the weather, should be protective finishes in the sense of retarding the 

movement of moisture. Even if the papreg itself requires no protection 

against weathering, it seems likely that a nonprotective coating probably 

would not maintain its own integrity very long. The Forest Products Laboratory 

1This mimepograph is one of a series of progress reports prepared by the Forest 

Products Laboratory to further the Nation;s war effort. Results here reported are preliminary and may be revised as additional data become available 

Mimeo. No. 1598 -1-
method of measuring moisture-excluding effectiveness of wood finishes is 

adaptable to similar evaluation of finishes on papreg. 

Testing Procedure 

Test specimens of papreg were 1/16 by 4 by 8 inches in size. The papreg 

was from batch number P1823-11-250; it, was made of paper impregnated with 

Bakelite resin BV-16526 by Consolidated Water Power and Paper Co. and laminated 

at the Forest Products Laboratory, using the same resin and a pressure of 250 

pounds per square inch. Plywood specimens were 3/32 by 4 by 8 inches in size, 

of several species of wood, made of three plies glued with phenolic resin, 

hot-pressed glue. All specimens were kept in 65 percent relative humidity at 

80° F. until they came to constant weight. 

Finish was applied to all surfaces of test specimens including the edges, and was allowed to dry in 65 percent relatilve humidity at 80°F. The specimens 

were then reweighed and the increase in weight recorded as the weight of finish, 

reported in pounds per 100 square feet of total surface. The total surface of 

the plywood specimens was 0,360 square foot, of papreg specimen 0.455 square 

foot. 

After corning to equilibrium at 65 percent relative humidity, the 

finished specimens, together with two specimens of papreg and two of each kind 

of plywood left unfinished to same as control specimens, were moved to 97 

percent relative humidity at 80° F. After one week in the high humidity, all 

specimens were again weighed. The increase in weight in 97 percent humidity 

was considered moisture absorbed by the specimen, and the difference in absorption between a finished specimen and the average for tho two unfinished 

control specimens of the same material was considered the moisture excluded 

by the finish. The moisture exluded by the finish, expressed in percentage 

of the average absorption by the unfinished control specimens, is the moistureexluding effectiveness of the finish, 

The extent to which the uncoated papreg and plywood specimens absorbed 

moisture during on0 week in 97 percent relative humidity, after previous 

equilibrium with 65 percent relative humidity, is shown in table 1. 

In applying finishing materials, wood filler was brushed on and then 

wiped off according to the usual practice, Aircraft dope was applied by 

brushing. All other finishing materials were applied by dipping. 

All but one of the finishing materials were commercial products sold 

for aircraft finishing. Comparison of one manufacturer's products with those 

of another, however, was not a primary objective of the work. The aim was 

to include finishes from a sufficient number of commercial sources to afford 

a reasonable representation. The major purpose was to study the combination 

Of finishing materials into finishing systems, particularly systems of minimum 

weight and minimum number of operations consistent with the degree of moistureexcluding effectiveness presumably required for adequate protection. It was 

Considered important to learn how much is contributed to the moisture-excluding 

Mimeo. No. 1598 -2-
effectiveness of a finishing system by sealers, fillers, surfacers, gloss

enamels, and camouflage enamels and to compare oleoresinous finishes with 

lacquer finishes and direct finishes with fabric finishes. The curtailment 

of the original program has impaired the scale on which these comparisons 

can be made. 

Accordingly, the finishing systems studied are not necessarily those 

recommended by the manufacturers of the products used. In general, emphasis 

was placed on systems consisting of fewer materials and fewer coats than are 

commonly advised. It was further assumed that a finishing system suitable for 

papreg may be somewhat simpler than the corresponding system for plywood, 

inasmuch as papreg may be considered sealed or primed to begin with. 

The one finishing material not obtained from a commercial source was 

an aluminum primer, which was made at the Forest Products Laboratory. A spar 

varnish was made from paraphenylphenol-formaldehyde resin, tung oil, and 

linseed oil, in conformity with Army-Navy Aeronautical Specification AN-TTV-118. To one gallon of the varnish, 1-3/4 pounds of standard aluminum paste 

was added. When freshly mixed, this paint dried promptly, but after standing 

for several days it became slow in drying. For these experiments it had 

stood some days before application. There is, therefore, reason to doubt that 

it is representative of good aluminum primers. 

The various classes of finishing materials used, such as sealer, surfacer, 

gloss enamel, and camouflage lacquer, are listed in table 2 together with 

identifying symbols and the Army-Navy Aeronautical or Army Air Forces specifications to which they presumably conformed. Materials were not tested for 

conformity to specifications, The identifying symbols as used in the tables 

were also used in marking test specimens. The symbols are combined to indicate finishing systems. Thus the symbol AXR means a system consisting of one coat of clear sealer, one coat of sanding surfacer, and one coat of camouflage 

enamel, applied in the order named. If more than one coat of a material is 

applied, the symbol is repeated for each additional coat; thus TT/TTTVV means 

a system consisting of two coats of clear dope, a covering of fabric, three 

coats of clear dope, and two coats of gloss pigmented dope, applied in the 

order named. 

Each manufacturer of finishing materials was assigned an identifying 

number, which appears in tables 3 ad 4. The key to the numbers is part of 

the original records of the work but is omitted in this report. For marking 

test specimens the manufacturer's number was written as a prefix and the 

identification number of the papreg or plywood from table 1 was written as a 

suffix. Thus the complete symbol 3-AXR-5 means finishing system AXR, using 

materials supplied by maufacturer 3, tested on yellow-poplar plywood. 

Results 

Table 3 presents the weight of finish and the moisture-excluding effectiveness for the direct-on-plywood or direct-on-papreg finishes tested. Table 4 

presents the results for fabric finishes, 

Mimeo. No. 1598 -3-
Papreg Compared with Plywood 

The moisture-excluding effectiveness of finishes on papreg varied 

through nearly the same range as finishes on wood. In table 3, the finishes 

on papreg varied from 9 percent to 74 percent in effectiveness. Finishes on 

plywood varied from -4 percent2 to 89 percent in effectiveness. In general, 

the finishes on papreg stood in the same order of effectiveness as they did 

on plywood, that is, the higher of two finishes on papreg was likewise the 

higher of the same two on plywood. 

The effectiveness of a finish on papreg increased with the thickness of 

the coating, just as has long been known of finishes on wood. Compare, for 

example, finish 8-M with 8-MM, finish 12-P with 12-PP, and finish 4-L with 

4-LL in table 3. In each comparison, the one-coat finish is lower in effectiveness, lighter in weight, and thinner than the corresponding two-coat finish. 

The one-coat finishes averaged 0.85 pound per 100 square feet and 29 percent 

effectiveness, whereas the two-coat finishes averaged 2.70 pounds per 100 square 

feet and 41 percent effectiveness. 

Less finish was applied on papreg than on plywood, even when the number 

of coats was the same on both kinds of material. The difference arose chiefly 

with the first coat of finish applied, which was usually much lighter in weight 

on papreg than on plywood. The relatively nonabsorptive nature of the surface 

presented by papreg, in contrast to the absorptive surface of wood, accounts 

readily for the observed difference in weight of finish applied. For the same 

reason, a single application of finish on papreg often formed a coating continuous enough to afford substantial moisture-excluding effectiveness, an 

achievement rarely possible on wood. Finish 12-P, for example, wits 48 percent 

effective on papreg, with a weight of 1.20 pounds per 100 square feet, and 

coating 3-S was 58 percent effective with a weight of 0.63 pound per 100 square 

feet. No one-coat finishes were tested on plywood because much previous work 

has shown that a single coat is nearly always low in effectiveness (Forest 

Products Laboratory Technical Note No. 181 ad Mimeograph No. R974). 

Finishes 1-BR, 8-ZM, 12-PP, and 12-RP were tested on papreg, birch, and 

Yellow-poplar. On papreg the four finishes averaged 40 percent effectiveness 

and 1.89 pounds per 100 square feet in weight. On birch the average effectiveness was 44 percent and the average weight 2.38 pounds. On yellow-poplar the 

average effectiveness was 34 percent and the average weight 2.69 pounds. The 

limited amount of data suggest that, on papreg, the weight of finish required 

to attain moisture excluding effectiveness between 44 percent and 34 percent 

may be fully 0.5 pound per 100 square feet less than is needed on plywood. 

This is a saving of one-sixth to one-fifth of the weight of the simpler types

of direct finish, 

Twelve of the finishes reported in table 3 were tested both on papreg 

and on yellow-poplar; their average weight and average effectiveness, respectively, were 1.80 pounds and 40 percent on papreg and 2.48 pounds and 24 percent 

2

Negative effectiveness means that the specimens with finish absorbed more moisture in 97 percent relative humidity than the control specimens without 

finish. Such results are usually considered experimental errors attributable 

to variability even among matched specimens of wood. 

Mimeo. No. 1598 -4-
on yellow-poplar. Seven finishes were tested both on papreg and on birch; their 

average weight and average effectiveness, respectively, were 2.27 pounds and 

40 percent on papreg and 2.69 pounds and 49 percent on birch. These results 

confirm the conclusion that a given degree of moisture exclusion can be attained 

on papreg with less weight of finish than is required on wood, 

From the data presented in the two preceding paragraphs it nay be that 

to attain equal effectiveness, slightly more finish is required on yellow-poplar 

than on birch. There were 12 finishes tested both on birch and on yellow-poplar. 

On birch the average weight. of the 12 finishes was 3.34 pounds per 100 square 

feet and the average effectiveness was 42 percent, whereas on yellow-poplar 

the average weight was 3.85 pounds and the average effectiveness was 36 percent. 

Among the fabric finishes from manufacturer 13, reported in table 4, wood 

sealer was applied for the first step on plywood but was not used on papreg. 

For each fabric finish on papreg, however, there was a corresponding finish 

on birch and on yellow-poplar plywood that differed only in the use of sealer 

on the plywood and its omission on the papreg. For the six sets of fabric 

finishes that may be so compared, the average weight of finish was 9.16, 8.81. 

and 9.47 pounds per 100 square feet on papreg, birch, and yellow-poplar 

respectively, and the average moisture-excluding effectiveness was 19 percent 

on papreg, 45 percent on birch, and 33 percent on yellow-poplar. The weight of 

fabric finish on papreg, therefore, was not significantly less than that on 

plywood, despite the omission of sealer on papreg, but the moisture-excluding 

effectiveness was much lower on papreg than on birch or yellow-poplar plywood. 

Apparently the oleoresinous sealer used on plywood must be credited with 

effecting the higher moisture-excluding effectiveness observed on plywood in 

comparison with papreg. If so, it is evident that the fabric and dope finishes 

offered much less resistance to movement of moisture than is obtainable with 

oleoresinous finishes, 

Comparisons of Finishing Systems 

Because of the limited number of tests, it is not possible to draw firm 

conclusions about the comparative effectiveness of the different finishing 

systems. The trends of the results, however, agreed well with expectations 

based on past experience with the kinds of finishing materials in question. 

Among one-coat finishes on papreg, an aluminum enamel was 58 percent 

effective, three gloss oleoresinous enamels averaged 43 percent effective, a 

gloss lacquer was 23 percent effective, six camouflage oleoresinous enamels 

average 20 percent effective, and two camouflage lacquers averaged 15 percent

effective. For systems consisting of two coats of a single material, aluminum 

enamel was 72 percent, gloss oleoresinous enamel 63 percent, gloss lacquer 31 

percent, and camouflage lacquer 29 percent effective, Three coats of gloss 

pigmented dope were 17 percent effective. 

Camouflage oleoresinous enamels generally were lower in effectiveness 

than gloss enamels although the poorest gloss enamel was less effective than 

the best camouflage enamel. Similarly, camouflage lacquer was less effective 

than gloss lacquer. The relatively permeable nature of the camouflage coatings 

Mimeo. No. 1598 -5-
results, of course, from the excessive proportion of pigment necessary to produce a lusterless surface. One camouflage enamel, however, was much more 

effective than the others. The most effective one, that made by manufacturer 

7, was not entirely lusterless and might not have passed specifications with 

respect to lack of gloss. The relatively effective camouflage enamel undoubtedly 

contained less pigment than the others. The results indicate a need for thorough 

study of the pigmentation of lusterless finishes for exterior surfaces. 

It is noteworthy that camouflage enamels, except for the unusual one of 

manufacturer 7, tended to have lower effectiveness on plywood than on papreg, 

whereas gloss enamel when applied in two coats was fully as effective on plywood 

as it was on papreg. Often two coats of camouflage enamel on plywood were less 

effective than one coat on papreg. Wood, of course, absorbs part of the vehicle 

from at least the first coat of a pigmented material, thereby materially increasing the proportion of pigment in the coating left on the surface. With camouflage enamel, which is already unduly high in pigment, apparently the second 

coat may he similarly altered by absorption of vehicle. Gloss enamel, however, 

has enough vehicle to prime the wood with the first coat, if it is a reasonably 

generous one, so that the second coat remains undisturbed in composition. The 

nonabsorptive nature of papreg, on the other hand, leaves even camouflage 

coatings essentially unaltered in composition. 

In line with the foregoing considerations, use of sealer under camouflage 

finishing coats sometimes improved the moisture-excluding effectiveness but it 

did not improve the effectiveness of gloss finishing coats. When a sanding 

surfacer was used under finishing coats, the weight of coating was increased 

greatly: the effectiveness usually increased observably but not to an extent 

comparable with the increase in weight. 

Among the fabric finishes, those with camouflage dope were significantly 

lower in moisture-excluding effectiveness than those with gloss pigmented dope. 

Use of aqueous-emulsion lacquer in place of aqueous-emulsion dope for cementing 

and sealing the fabric slightly improved the effectiveness at the cost of slight 

increase in weight. The improvement in effectiveness may have been no more 

than the increased weight accounts for. In view of their relatively great 

weight, all of the fabric finishes must be considered very low in effectiveness 

per unit weight. 

Minimum Finishes for 50-percent Effectiveness 

The minimum degree of moisture-excluding effectiveness that should be 

demanded of protective finishes for wood exposed to the weather has not been 

established, but for aircraft a minimum of 50 percent certainly should not be 

unreasonable. Good house paints materially exceed that minimum. 

Table 5 summarizes the finishes that attained at least 50 percent 

moisture-excluding effectiveness on papreg and on yellow-poplar plywood. The 

finishes are listed separately for papreg and for plywood, in each starting 

with the finishes of fewest costs and lightest weight and proceeding to the 

more complicated and heavier finishes. The last column of table 5 records 

the ratio of moisture-excluding effectiveness to weight of coating. The ratio 

is useful as a crude indication of a relation between effectiveness and weight, 

Mimeo. No. 1598 -6-
but a more satisfactory scale of values would be furnished by the ratio between 

moisture impedance and the product of thickness of coating by average specific 

gravity of coating. 

On papreg the arbitrary minimum of 50-percent moisture-excluding effectiveness was achieved with a single coat of aluminum enamel or of gloss enamel, 

but no one-coat finish tested on plywood reached that standard. On plywood, the minimum was attained in two-coat systems having either aluminum enamel 

or gloss enamel for tho second coat. For finishes with lusterless surface, the 

fewest coats to attain 50-percent effectiveness were two on papreg and three on 

plywood. 

None of the lacquer, dope, or fabric-and-dope finishes tested attained 

50-percent moisture-excluding effectiveness on papreg or on yellow-poplar 

plywood. 

On papreg the minimum of 50-percent effectiveness was attained with an 

aluminum finish at a weight of 0.63 pound per 100 square feet, but yellowpoplar plywood took 1.49 pounds of alminum finish to pass the required minimum 

in effectiveness. Similarly with gloss finish, a weight of 1.39 pounds sufficed 

on papreg whereas 3.50 pounds were necessary on yellow-poplar plywood to attain 

50 percent effectiveness. Among lusterless finishes, tho lightest to pass the 

requirement on papreg weighed 4.03 pounds and on yellow-poplar plywood weighed

7.30 pounds per 100 square feet. 

Greatest moisture-excluding effectiveness for least weight (highest ratio 

Of E/W) was achieved with aluminum finishes, both on papreg and on plywood. 

Next in order were the gloss finishes in which the sanding surfacer was omitted. 

A surfacer always increased the weight of finish without achieving a proportionate increase in moisture-excluding effectiveness. All of the lusterless 

finishes that met the required minimum of 50 percent moisture-excluding effectiveness were relatively heavy finishes. 

Mimeo. No. 1598 -7-
Table 1.--Moisture absorbed by uncoated papreg and plywood test 

specimens after 7 days in 97 percent relative humidity. 1 

Mimeo. No. 1598 
Table 2.--Classes of finishing materials used. identification symbols, 

and specifications to which they presumably conformed. 

Mimeo. No. 1598 
TABLE 3 - WEIGHT AND MOISTURE-EXCLUDING EFFECTIVENESS OF DIRECT FINISHES 
TABLE 4 - WEIGHT AND MOISTURE-EXCLUDING EFFECTIVENESS OF FABRIC AND DOPE FlNlSHES 

Z M 52273 F 
Table 5.--Summary of finishes affording at least 50 percent moisture-exluding 

effectiveness on papreg and on yellow-poplar plywood. 

Mimeo. No. 1598